ABSTRACT
Van de Hulst's localization principle relates the principal mode number to the external beam position that maximizes energy coupling to a spherical cavity mode. Our experiments in lasing microdroplets verify localization but only for low- Q modes, when the cavity may be considered to be a nearly perfect homogeneous sphere. The principle fails in the perturbation-dominated high- Q limit. Surprisingly, near-surface resonances are still efficiently excited in these cases but require impact parameters slightly smaller than the sphere radius. Numerical modeling suggests that this new input channel depends on surface scattering.
ABSTRACT
Spectrally resolved emission (270-560 nm) from dilute suspensions of washed Bacillus subtilis and Escherichia coli were measured by use of tunable laser excitation between 270 and 300 nm. Integrated absolute emission cross sections increase with decreasing excitation wavelength and range from 1.8 x 10(-12) to 6.0 x 10(-11) cm(2)/(particle sr). An emission band near 340 nm dominates all observed spectra. At each excitation wavelength spectrally resolved emissions from the E. coli and B. subtilis suspensions are indistinguishable.
ABSTRACT
Continuous-wave stimulated Raman scattering was observed in 11-13-microm-diameter benzene and toluene microdroplets at pump intensities as low as 8 and 24 kW/cm(2), respectively. Low thresholds were achieved by exploiting simultaneous pump and Stokes wave resonance in the droplets and Raman gains that were cavity QED enhanced ~50 times with respect to bulk liquid values. Based on a photon-state conservation argument, the cavity gain enhancement factor may be approximated by the ratio of the spectral spacing between resonant modes of the same order to that of the homogeneous Raman linewidth. This relation appears to be consistent with the relative experimental behavior of benzene, ethanol, and toluene.
ABSTRACT
Recent advances in an aerosol-generation technique have permitted the accurate identification of optical resonance-mode features of micrometer-sized freely falling droplets for several different optical processes. Both input and output resonant features of fluorescence and lasing from dye-doped microdroplets were assigned to specific spherical cavity modes by using two independent procedures: (1) by matching observed fixed-angle elastic laser light scattering as a function of droplet size to calculated scattering intensities from the Lorenz-Mie theory, and (2) by matching observed resonance peaks to computed cavity-mode positions by automated correlation. Agreement between these two complementary techniques establishes high confidence in the resulting mode identifications. Assignments of observed emission peaks provide insight into droplet-emission mechanisms.
ABSTRACT
The effect of absorption on microdroplet resonance emission line intensities was studied in 15-microm-diameter Rhodamine 6G/ethanol solution droplets. Absorption was controlled by varying the concentration of the additive nigrosin. Spectrally integrated intensities of resonant features are found to be proportional to a droplet cavity mode efficiency Q(a)/(Q(a)+Q(o)) expressed in terms of cavity output coupling and absorption factors Q(o) and Q(a), respectively. These Q's are determined from linewidths calculated from Lorenz-Mie theory by using combinations of the real and complex indices of refraction. An experimental upper limit of Q for first-order modes was determined to be 10(8) from the data.
ABSTRACT
Spectral features observed in stimulated Raman scattering from 20-microm methanol droplets at 1-GW/cm(2) pumping levels appeared locked in wavelength at the center of their respective gain profiles. Typically, a feature's wavelength remained constant while the droplet size varied by as much as 0.3%. This is attributed to frequency-pulling effects caused by the presence of Raman gain. Wavelength shifts of up to +/-0.5 nm in the positions of morphologydependent resonances would explain the observations and are consistent with calculations.
ABSTRACT
Nonlinear Mie scattering at 532 nm was observed from 23-microm-diameter ethanol droplets excited by picosecond light pulses. Low-order optical modes (l = 2-4), normally hidden at low intensity, became visible as new peaks in elastic scattering spectra at 2 GW/cm(2). When higher-Q resonances were selectively excited, the scattered light showed an amplitude-modulated time dependence. The data are explained by electrostrictive generation of acoustic modes of the droplet by the incident radiation and subsequent coupling of the acoustic disturbance back onto the light field.
ABSTRACT
Experimental elastic-scattering characteristics of layered microspheres are reported. Single particles consisting of glass cores coated with glycerine are suspended in an evacuated quadrupole trap, and scattered light intensity is measured at a fixed angle as the glycerine evaporates. The results are compared with those calculated for concentric spheres using the Mie theory. Excellent agreement is obtained for glycerine layers of thickness less than 700 nm. For thicker layers, differences occur that are attributed to the effect of gravity on particle structure.
